dir.c: remove dead function fnmatch_icase()

It was largely replaced by fnmatch_icase_mem() and its last use was in
84b8b5d (remove match_pathspec() in favor of match_pathspec_depth() -
2013-07-14).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>